1. Field of the Invention
The present invention relates to a method for reducing NOx emissions from rotary preheater mineral kilns, and particularly from rotary preheater lime kilns, by coupling the temperature control and gas composition afforded by high temperature mixing systems with the injection of nitrogen containing chemical additives at a predetermined location and within an optimal temperature window.
2. Description of the Prior Art
A variety of mineral processing techniques utilizing high temperature kilns are known and commercially practiced at the present time. These commercial processes, particularly lime kiln processes, involve a number of different basic kiln designs and modifications thereto, diverse fuels and varying degrees of automation. Among the basic types of kilns employed for the production of lime in the United States at the present time are the straight rotary kiln, the rotary preheater kiln, and the vertical shaft kiln. In the conventional straight rotary kiln, a long straight cylindrical shaft is built on a slight incline. Limestone is charged into the kiln at the elevated end and advanced toward a flame at the lower end of the shaft where the limestone is calcined into quicklime and discharged. In addition, a flow of combustion gases that are derived from fuel injected at the lower end of the shaft move countercurrent to the downward flow of lime.
Vertical kiln designs may be the most widely employed designs in the world. There are hundreds of modifications to the basic vertical layout. However, all such designs tend to have four distinct zones: (1) a stone storage zone, which is a vertical or often a modified hopper-shaped zone; (2) a preheating zone, designed to heat the stone near dissociation temperatures; (3) a calcining zone, where combustion occurs; and (4) a cooling and discharge zone, usually shaped like an inverted, truncated cone, with the quicklime being discharged from the bottom of the cone.
The rotary preheater kiln design is an attempt to improve thermal efficiency in the overall limestone calcining operation. Shaft preheaters, which often resemble a vertical kiln looming above and astride the inlet end of a rotary kiln, have been the most frequently installed preheaters.
Whatever, the particular kiln design, undesirable gases, such as nitrogen oxide (NOx), are produced during the heating and cooling processes that takes place inside the kiln. Growing concerns regarding health and environmental issues surrounding such emissions from combustion processes generally have given rise to several technologies developed to reduce NOx emissions. These technologies have been applied in a variety of different situations which extend far beyond the field of mineral calcining operations. In the case of unflued gas burners used in the home, for example, technologies such as the “Bowin low-NOx technology” are utilized, where air and fuel gas are premixed at a ratio greater than or equal to the stoichiometric combustion requirement. Other technologies, such as selective catalytic reduction and selective non catalytic reduction reduce post combustion NOx in a variety of different industrial settings. In the consumer arena, the so-called “catalytic converters” have been widely used in recent years on automobile engine exhausts to reduce NOx emissions.
In the industrial arena, there are current methods for reducing NOx emissions by injecting ammonia or ammonia precursors which have been used successfully in selective non catalytic reduction systems for control of NOx emissions from boilers in the power industry. However, their effectiveness is often limited by the extent to which flue gas temperature at the injection point remains within a relatively narrow temperature window, somewhere on the order of 1600° F.-2200° F. (871-1204° C.). Normal daily load variations in the industry will frequently drive the temperature out of this range. In contrast, such temperature excursions are far less frequent or as wide-ranging in mineral processing applications such as kilns used to produce quicklime.
Thus, the possibility of using ammonia injection to reduce NOx emissions seems to be a feasible approach for mineral processing kilns. However, both the straight rotary and vertical shaft kilns described above have an optimum temperature for the injection of the urea or ammonium which is located approximately in the middle of the kiln. This adds a degree of difficulty to the injection process and limits its usefulness as a NOx reduction technique for such kiln designs.
In addition to the injection of ammonia as a means of controlling NOx emissions in mineral processing kilns, another known technique utilized in rotary mineral kilns generally deals with the introduction of turbulent gases into the kiln interior. Controlled high temperature mixing of the gases within the kiln provides a means of reducing temperature and compositional stratification, resulting in more efficient heat transfer and improved fuel consumption within the kiln.
There is existing patent literature that provides an explanation of the technique of mixing high temperature gases in mineral processing kilns in order to reduce NOx emissions. For example, U.S. Pat. No. 6,672,865, issued Jan. 6, 2004, teaches the injection of air with high velocity/high kinetic energy into a mineral kiln to reduce or eliminate stratification of kiln gases and thereby further reduce NOx emissions. However, this reference does not teach the concept of further enhancing NOx reduction by coupling the temperature control and gas composition afforded by high temperature mixing with the use of ammonia or ammonia precursors which are introduced at a particular location in the process and within an optimal temperature window.
There are also known kiln arrangements for treating fine-grained material such as cement clinker in which exhaust air from the cooling zone is introduced in two branch air streams at different points into the calcination zone, the delivery of air and fuel being set in such a way that the section of the calcination zone which is operated under oxidizing conditions and which adjoins the section operated under reducing conditions. These technologies attempt to achieve a lowering of the basic level of NOx through a targeted adjustability of the reaction conditions in the calcination zone, particularly by the choice of an optimal temperature window and a favorable oxygen content. While these known technologies attempt to achieve targeted reaction conditions in the calcination zone, they do not appear to specifically be concerned with combining the high velocity/high kinetic energy air delivery with the injection of an ammonia additive.
Much in the same way, known techniques describe a fluid bed of particle material which is fluidized by the use of primary air and which may have a reducing agent further introduced into a combustion zone, along with the injection of secondary air, to suppress NOx emissions. These prior art techniques again teach adjusting the quantities of air supplied to the calcining or combustion chamber in stoichiometric fashion in order to provide either a reducing or oxidizing atmosphere. This does not necessarily equate to the use of high velocity/high kinetic energy air injection, however, and these existing technologies do not combine the use of nitrogen-containing chemical additives.
Despite the fact that various technologies have been developed to address the presence of NOx emissions in a variety of commercial and industrial settings, a need continues to exist for further improvements in lime kiln operations due to the nature of the combustion processes which take place inside a limestone kiln.
A need accordingly exists for a method to further enhance NOx reduction in rotary preheater limestone kilns and to overcome the other described problems with respect to the prior art, which improved method will become apparent to one skilled in the art from the detailed description provided below.